Stopgap fix for finding module for a file mapped in the VFS

If we lookup a path using its 'real' path first, we need to ensure that
when we run header search we still use the VFS-mapped path or we will
not be able to find the corresponding module for the header.

The real problem is that we tie the name of a file to its underlying
FileEntry, which is uniqued by inode, so we only ever get the first name
it is looked up by. This doesn't work with modules, which rely on a
specific file system structure.  I'm hoping to have time to write up a
proposal for fixing this more permanently soon, but as a stopgap this
patch updates the name of the file's directory if it comes from a VFS
mapping.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@209534 91177308-0d34-0410-b5e6-96231b3b80d8